1. Technical Field
The invention relates to pressurized water nuclear reactors of the type having a core formed by fuel assemblies of prismatic shape disposed side by side inside a cylindrical core casing having a vertical axis and defining, between the core and the casing, a space in which a solid partitioning structure is disposed.
2. Prior Art
Nuclear reactors of the above-defined type are already known. As compared with conventional partitioning, formed of a set of metal sheets, called "baffle plates", forming a polyhedron surrounding the core, and of formers distributed over the height of the core, solid partitioning is a better neutron reflector since the partitioning structure is then formed essentially of materials, such as iron, reflecting the neutrons better than water.
Different solid partitionings are known. European No. 051,390 describes a solid partitioning structure comprising columns disposed side by side and each formed of stacked modular blocks, secured to the lower core supporting plate by tie rods. The modular blocks are secured to each other by vertical keys, in an arrangement similar to that used in the graphite stacks of graphite-gas reactors. The modular blocks are cooled by water circulation in vertical channels formed by mutually aligned holes in the blocks.
That construction has drawbacks. Since the partitioning structure is not connected to the core casing, it is not centered accurately with respect to the casing. The water in the large number of cooling channels, required for limiting the stresses and deformations of thermal origin, greatly reduces the reflecting power of the partitioning and the water flow short-circuiting the core reduces the temperature of the coolant at the outlet of the reactor. The clearances between the modular blocks are swept by a water flow in addition to that passing through the channels.
A more advantageous partitioning device is described in European No. 0,117,768; it is formed of juxtaposed modular blocks fixed to the core casing and separated from each other by thin wedges. With this arrangement, the cooling water flow through the partitioning may be accurately adjusted. But this solution is still not perfect, particularly because it uses a large number of assembling screws subjected to high stresses and because clearances may exist between the blocks located in the proximity of the fuel assemblies and give rise to water jets directed towards some of the fuel rods and inducing vibrations.